Many innovations for optical communication systems have involved the manner in which light waves are switched and manipulated. In many optical transmission applications, it is necessary to perform one or more of the following actions on light: switching, attenuation, routing to different locations or manipulating the phase of light. Such actions are critical for realization of the optical networks that are the foundation of global communications systems.
Optical communication systems increasingly employ wavelength division multiplexing (WDM) techniques to transmit multiple information signals on the same fiber, and differentiate each user sub-channel by modulating a unique wavelength of light. WDM techniques are being used to meet the increasing demands for improved speed and bandwidth in optical transmission applications. In optical communication networks, such as those employing WDM techniques, individual optical signals are often selectively routed to different destinations. Thus, a high capacity matrix or cross-connect switch is often employed to selectively route signals through interconnected nodes in a communication network.
At the heart of these cross-connect switches is the single switching unit. Single switching units should exhibit low manufacturing and operation costs, small losses of the optical signal when passing through the switch (low insertion loss), and high blocking of unwanted signals (high extinction ratio). Many switches used in optical communication networks are manual, and are relatively cheap to manufacture, but expensive to operate. In addition, available switches tend to prevent high switching speed and flexibility. Electronic switches first convert the optical signal into an electronic signal, perform the switching and then convert back into optical signals. These conversions are very expensive and the switches are complex to manage but allow considerable flexibility. As networks grow and become dense, however, electronic switches become increasingly expensive and harder to fabricate.
Therefore, optical switches that operate directly on the light wave are favorable. Optical switches are often realized in optical waveguides that can be manufactured with low cost and enable easy multiplexing and de-multiplexing of the WDM signal using waveguide grating routers (WGR). For a detailed discussion of waveguide grating routers, such as optical star couplers, see U.S. Pat. No. 4,904,042 to Dragone. Switching in waveguides is often accomplished by applying phase or amplitude changes using an electrooptic effect or a thermooptic effect. The electrooptic effect usually requires special and expensive waveguide materials, such as InP or LiNbO3, that exhibit nonlinear effects and are used for fast switching and specialized applications. Thermooptic switching (a heat induced change in the index of refraction) in waveguides is robust and is extensively used in combination with WGR in optical waveguide circuits. However, thermooptic switches suffer from high power consumption and limit the complexity of circuits that can be built due to thermal crosstalk and maximum power limitations.
Recently, micro electro mechanical systems (MEMS) switches have been introduced for network applications. MEMS switches are usually movable mirrors that change the propagation direction of light, or block light. For a discussion of a wavelength-selective add-drop multiplexer that uses movable mirrors to add and/or drop spectral components from a wavelength-division-multiplexed optical signal, see, for example, U.S. Pat. No. 5,974,207 to Aksyuk et al, assigned to the assignee of the present invention and incorporated by reference herein. To change the propagation direction of the light, or block the light, a shutter must be moved a distance long enough to move the shutter in and out of a light beam or tilt the shutter with an angle larger than the angular width of the optical beam. These displacements are usually challenging to make with MEMS actuators that excel at microscopic motion. If switching can be achieved by motion that is the size of the optical wavelength (about 1-2 μm for common communications systems), MEMS switches could be implemented in waveguides and other systems.